1. Field of the Invention
The present invention generally relates to a compensator for reactive power (hereinafter referred to as "var") provided in an electric power system, and particularly relates to a static var compensator made up of static devices such as thyristors or the like.
2. Description of the Prior Art
At various points in an electric power system, there are provided devices for controlling voltages/var to predetermined values so as to allow the electric power system to operate stably. For example, in a power station, there is provided an automatic voltage control apparatus for making a terminal voltage of the power station constant. In a substation in a neighborhood of customers, there is provided a voltage/var control apparatus constituted by phase modifying equipment such as a tapped transformer, a static capacitor, a reactor and the like so as to control the voltage at the customers locations is that it is constant or to reduce the transmission loss in the electric power system. In such conventional phase modifying equipment, however, there have been disadvantages in that only stepwise control can be performed because the equipment is connected to an electric power system through a mechanical switch, and the equipment is unsuitable for use in a major electric power system because its capacity cannot be made large. In order to eliminate the foregoing disadvantages, recently, various static devices using a thyristor bulb suitable for use in a major electric power system have been used practically.
These devices, however, have a large effect on an electric power system because of their large capacity, and therefore it is necessary to make these devices contribute to the stable operation of the electric system. For example, in a static var compensator provided in a major electric power system, var cannot be detected so as to be directly controlled unlike a static var compensator provided in a terminal substation of the electric power system. Therefore var is indirectly controlled such that a voltage at the point where the var compensator is provided is detected and the detected voltage is controlled to a predetermined value. It is known that higher harmonics (particularly, the second-order higher harmonics) are superimposed on the voltage of the electric power system. In the static var compensator provided in a major electric power system, control is performed so that a deviation of a detected value of a voltage, the mean value of which is affected by the higher harmonics from a predetermined setting voltage, is derived, and a firing angle of a thyristor bulb is controlled through a voltage control circuit having a high control gain, so that in opposition to an expected object, the var becomes large. A static compensator with large capacity, however, does not have the disadvantages as described above. Further, upon occurrence of a fault in an electric power system, a terminal voltage of the static var compensator is lowered so that the thyristor bulb cannot be fired. It is therefore desirable that when a breaker is opened (or the fault is removed), the control function of the static var compensator is rapidly restored so as to reduce over-voltage of the electric power system which is generated when the fault is removed.
The large-capacity static var compensator (hereinafter, simply abbreviated to "SVC") of the kind as described above is disclosed, for example, in NISSIN ELECTRIC TECHNICAL REPORT, Vol. 28, No. 3, 1983, or in Article entitled "Simulator Test for Stabilizing System by SVC using Digital Control Apparatus" (PAPERS OF TECHNICAL MEETING ON POWER ENGINEERING, July 25, 1985 (PE-85-7 IEE JAPAN)).
The conventional SVC controlling method executed when a fault is generated in an electric power system is such that the equipment is stopped or an output for controlling the equipment is made to be zero (see the above Article, page 65, lines 6-7), and the point that the equipment is continuously operated to the utmost so as to maintain the electric power system has never been taken into consideration. Further, any reference fails to disclose the influence by the higher harmonics.